1. Field of the Invention
This invention relates to cross-linked starches that are useful as fluid loss control additives for aqueous-based subterranean treatment fluids, such as drilling, workover and completion fluids.
2. Related Background Art
The cross-linked starches of this invention may be advantageously used in oil field applications. Particularly, the starches may be incorporated into fluids used in operations where there is contact with a subterranean formation. Drilling, workover, and completion fluids are examples of fluids used in subterranean formations.
Drilling fluids may be used for any of several functions that allow evaluating or producing a reservoir (formation) for oil, gas, or water, The drilling fluid may be pumped into the wellbore during the drilling operation to cool the drill bit and to flush out the rock particles that are sheared off by the drill bit. A "drill-in" fluid is often used while drilling the production zone.
Workover fluids may be used to perform one or more of a variety of remedial operations on a producing oil well with the intention of restoring or increasing production. Examples of workover operations include, but are not limited to, deepening, plugging back, pulling and resetting a liner, squeeze cementing, shooting and acidizing.
Completion fluids may be used to perform one or more of a variety of oil field applications illustrated by, but not limited to, operations such as cementing, using spacers, perforating, gravel packing, installing casing, underreaming, milling and a variety of simulation techniques such as acidizing and the like.
Subterranean treatment fluids are used in well operations, particularly oil well operations, for various purposes. The subterranean treatment fluids are generally prepared at the well site by admixing a viscosifying agent and a base fluid. The viscosifying agent thickens or viscosities the base fluid, thereby increasing the ability of the fluid to suspend or flush out the rock particles. The subterranean treatment fluid may also advantageously contain other additives that are conventionally used in well treatment operations, as needed, based upon the specific site requirements and environmental conditions.
A common problem associated with the use of subterranean treatment fluids is the loss of fluid into the surrounding formation near the wellbore. Fluid loss control additives are added to the subterranean treatment fluids to limit exposure of the formation and also control leak off of the liquid components to the surrounding subterranean formation. As a result, the subterranean treatment fluids that are most useful in well operations possess adequate high water retention capacity. Desirably, the subterranean treatment fluid should retain high water retention capacity under the often adverse environments encountered during use. For example, high temperature conditions are encountered in deep wells, where operating temperatures frequently exceed 250.degree. F. Low temperature conditions are encountered in shallow wells or in areas of a well that are closer to the earth's surface. High salt conditions are created when brine-containing subterranean treatment fluids are used. Accordingly, the fluid loss control additive used in subterranean treatment fluids should preferably be stable in both high temperature and high salinity environments. More preferably, the fluid loss control additive should be stable over a range of temperatures and should function in environments of either high or low salinity.
Natural starches are a well known and important class of materials useful as fluid loss control additives. However, it is also well known that starches do not possess long term stability and tend to degrade when maintained at elevated temperatures. For example, at temperatures in excess of 225.degree. F., natural or conventional starches begin to degrade, and will fail to provide adequate fluid loss control.
Several approaches have been used to increase the stability of starches to provide more stable well drilling fluids. For example, U.S. Pat. No. 4,090,968 discloses the use of quaternary ammonium starch derivatives as fluid control additives that are stable at high temperatures. These derivatives were prepared by reaction of starch with epichlorohydrin and a tertiary amine.
A thixotropic three-component well drilling fluid, consisting of a cross-linked potato starch, a heteropolysaccharide derived from a carbohydrate by bacteria of the genus Xanthomonas, and hydroxyethyl cellulose, providing improved water loss control is disclosed in U.S. Pat. No. 4,422,947.
U.S. Pat. No. 4,652,384 discloses the use of selected cross-linked starches to provide fluid loss control at elevated temperatures. The starch, which is cross-linked to a rather high degree under specified conditions, requires activation at elevated temperature for over four hours in order to achieve suitable effectiveness.
Other well treating fluid blends have been prepared by incorporating Xanthomonas gum and an epichlorohydrin cross-linked hydroxypropyl starch, as described in U.S. Pat. No. 4,822,500. This particular combination of additives interact synergistically to enhance suspension characteristics and decrease fluid loss.
U.S. Pat. No. 5,009,267 discloses fluid loss control additives for fracturing fluids composed of blends of two or more modified, or cross-linked, starches or blends of one or more natural starches with one or more modified starches.
Although many of the cross-linked starch compositions described above offer improvements over conventional starch, there remains a need in the industry for a readily dispersible starch additive that can provide good fluid loss control over a wide temperature range and that is stable in brine-containing fluids.